1. Field of the Invention
This invention relates to a system and procedures therewith for determining concentration of electrolytes in the field of medical examination.
2. Description of the Prior Art
In the medical examination field, ions in the human blood, urine, and so on, in particular, positive ions such as sodium ion (Na.sup.+) and potassium ion (K.sup.+), and negative ions such as chloride (Cl.sup.-) ion are determined, and the obtained data is useful as diagnostic information. For the determination of ionic concentrations, an ion-selective electrode unit can be used comprising at least one negative electrode responsive to positive ions and/or at least one positive electrode responsive to negative ions and a reference electrode.
Once a sample of a test solution (this term refers hereinafter to a solution containing an electrolyte of which an ionic concentration is to be measured) such as blood is placed in such an ion-selective electrode unit, a positive or negative ion activates the corresponding negative or positive electrode to produce an electromotive force in accordance with the concentration of it between an activated electrode and the reference electrode.
When ionic concentration is determined as described above, the ion-selective electrode unit may permit the electromotive force generated by it to drift with the course of time. In order to avoid the unwanted influence and thus to make accurate determination, typically the ionic concentration of a test solution is determined with respect to a correction solution having substantially the same composition as the test solution. The potential level of the correction solution is measured before or after the ionic concentration measurement of the test solution, and thus the ionic concentration of the test solution is obtained as the difference of the measured potential level of the test solution from the measured potential level of the correction solution.
Correction solution for use in correcting the determination of ionic concentration of a test solution should be prepared or composed to have an always constant potential level Eo under normal conditions, and the ionic concentration of a test solution such as sample 1, 2 or 3 is determined in accordance with the difference in potential level from Eo as shown in FIG. 5. Eo may be measured before or after determination of the ionic concentration of a test solution, preferably alternately in measurement of test and correction solutions possibly with effective results.
There is a problem encountered in the determination of the concentration of electrolytes in the prior art if the potential level of correction solution, which originally should remain constant, varies for some cause, ionic concentration of test solution can be determined with error due to the variation, resulting in an inaccurate value.
FIG. 6 shows where Eo deviates by .DELTA.E for some cause between determinations with test solution samples 1 and 2. As a result, E.sub.2 is not obtained but E.sub.2 ' is obtained instead. Thus an error results.